The present invention relates generally to producing a nuclear particle beam of narrow energy spectrum and more specifically to producing a neutron beam for Boron Neutron Capture Therapy (BNCT). The present invention also relates to increasing the production rate of an accelerator based radionuclide provided that the target nuclide from which the radionuclide is generated has a high melting point and can be made into thin foil. Specific example is generation of palladium 103 from a beam of energetic proton bombarding a rhodium 103 target.
BNCT is potentially the most advanced cancer treatment which presently is at clinical stage for inoperable brain tumors and melanoma. Since this technique can selectively target and destroy tumor cells without injuring adjacent cells, future research will apply to many other types of cancer. The success of BNCT as an idealized form of therapy is dependent on two processes: the delivery of sufficient Boron 10 to a cancer tumor followed by bombardment of the tumor by low energy neutrons. The neutron flux on the tumor for a reasonable treatment time should exceed 10.sup.9 neutrons per cm.sup.2 per second. The only available source for this high intensity neutron remained to be a reactor based neutron source which is currently used for clinical trial. However, the neutron energy spectrum obtained from a reactor is not optimum and to provide sufficient neutrons at the treatment area a reactor has to operate at a very high operating power, on the order of several megawatts. This is a result of very large neutron spectrum in the core which requires a relatively long distance to slow down high energy neutrons using suitable moderators. The other drawbacks of a reactor are its very high price and the fact that it cannot be installed in hospitals for safety reasons.
In contrast to the reactor based neutron sources, accelerator based neutron sources for BNCT appear to have attractive features such as lower cost, reduced residual reactivity, much lower operating power, reduced safety concerns, and better neutron energy spectrum. Many designs have been proposed for using a beam of proton impacting lithium or beryllium targets and producing neutron through (p, n) reaction. In this reaction a proton collides with the nucleus of the target nuclide and causes emission of a neutron. With a proton beam of 2 to 4 MeV the calculated beam current to achieve adequate neutron flux in the treatment area varies somewhat and is estimated between 20 to 50 mA. Many complex issues in these designs remain to be solved. First, production of 20 to 50 mA beam of 2 to 4 MeV proton is technically challenging and remains to be unsolved. A potential solution to this problem is very costly as well. The other unsolved issue is in relation to the usage of lithium as a target which has a melting point of only about 181.degree. C. For example, a beam of 20 mA and 2.5 MeV proton produces 50 kW heat in the target. There is no practical solution for removal of this heat from a solid lithium target of acceptable area. Beryllium, on the other hand, has a much higher melting point, about 1250.degree. C. But a beryllium target has a lower neutron yield than a lithium target. The neutron yield of (p, n) reaction from beryllium target at 4 MeV becomes comparable to a lithium target at 2.5 MeV. Therefore, for producing the same amount of neutrons and by using the conventional method, a beryllium target needs higher power beam than a lithium target, by about a factor of 4/2.5=1.6.
With the present invention, however, the differences between power consumption for producing a 2.5 and a 4 MeV proton of the same current becomes insignificant. But the main objects of the present invention are to solve the production of high current beam for generation of neutrons and to provide an easy solution for dissipating the heat load from the beam in the target.
The neutron utilization efficiency is the ratio of the rate of useful neutrons in the treatment zone to the rate of total neutrons generated in the target or the reactor core. The useful neutrons are those with energy of several keV. Suitable moderators should be used to degrade the fast neutrons generated in the target to useful neutrons in the treatment zone. When the neutron energy spread where they are born is large a relatively large distance between their birthplace and the treatment zone should be filled with moderators to degrade the fast neutrons to the treatment regime. This in turn lowers the neutron utilization efficiency, since moderators that slow down the fast neutrons inevitably causes scattering and loss of other neutrons. When all neutrons born in the target have approximately the same energy they respond similarly to a moderator. With suitable choice of moderators they can be brought to the treatment zone with high utilization efficiency.
Neutrons generated with the present invention have approximately the same energy. Subsequently, a relatively high neutron utilization efficiency can be obtained with the present invention.
As has been mentioned at the beginning of this section, another object of the invention relates to increasing the production rate of an accelerator based radionuclide. In nuclear medicine certain types of radionuclides are used as therapeutic seeds. For example, palladium 103 is an x-ray emitting radionuclide that has a half life of 17 days. It is used for interstitial implantation as small seeds in tumors. The x-ray emitted from palladium 103 has a short range in tissues and subsequently is absorbed locally by tumors which results in gradual destruction of tumors. Palladium 103 can be produced either by neutron activation of palladium 102. Pd-102(n, .gamma.)Pd-103, by using neutrons from a reactor, or through a beam of proton from an accelerator, which is mostly a cyclotron, bombarding a rhodium 103 target. Rh-103(p, n)Pd-103. The energy of the proton beam for production of palladium 103 ranges from 12 to 16 MeV. The accelerator route has many clear advantages over the neutron route. For example, the accelerator route can provide a carrier free palladium 103 (free from other palladium isotopes) and with much higher specific activity than the neutron route.
One of the objects of the present invention is to increase the yield of palladium 103 from the rhodium by using the target system of the present invention and by keeping the proton beam at an energy where the excitation function of palladium 103 from rhodium 103 is maximum. This method may also be applied for production of other radionuclides provided that the target nuclides from which they are generated meet the requirements mentioned earlier in this section.